Anion binding behavior of heterocycle-strapped calix[4]pyrroles

A comparative study of the halide and benzoate anion binding properties of a series of phenyl, pyrrole, and furan-strapped calix[4]pyrroles has been carried out. These receptors, which have previously been shown to bind the chloride anion (Yoon et al., Angew. Chem., Int. Ed. 47(27):5038–5042, 2008), were found to bind bromide and benzoate anion (studied as the corresponding tetrabutylammonium salts) with near equal affinity in acetonitrile, albeit less well than chloride, as determined from ITC measurements or NMR spectroscopic titrations. This stands in marked contrast to the parent octamethylcalix[4]pyrrole, where the carboxylate anion affinities are substantially higher than those for bromide anion under identical conditions. This finding is rationalized in terms of tighter binding cavity present in the strapped systems. For all three anions for which quantitative data could be obtained (i.e., Cl^?, Br^?, PhCO₂ ^?), the pyrrole-strapped system displayed the highest affinity, although the relative enhancement was found to depend on the anion in question. In the specific case of fluoride anion binding to the pyrrole-strapped receptor, two modes of interaction are inferred, with the first consisting of binding to the calix[4]pyrrole via NH-anion hydrogen bonds, followed by a process that involves deprotonation of the strapped pyrrolic NH proton. A single crystal X-ray diffraction analysis provides support for the first of these modes and further reveals the presence of a methanol molecule bound to the fluoride anion.     

Calix[2]bipyrrole[2]furan and calix[2]bipyrrole[2]thiophene: new pyrrolic receptors exhibiting a preference for carboxylate anions

Heterocycles other than pyrrole, specifically bipyrrole, furan, and thiophene, have been used to construct two new, calixpyrrole-like anion receptors; binding studies, carried out by ITC in CH3CN, reveal a selectivity for "Y-shaped" anions, such as benzoate, over spherical ones, such as chloride.     

N-confused calix[4]pyrroles

This is a first review devoted to N-confused calix[4]pyrroles (NCCPs). NCCPs are a relatively recent arrival to the family of the pyrrole-based anion binding macrocycles, being for the first time identified in 1999. Yet, in a relatively short time these calix[4]pyrrole (CP) isomers attracted attention of the community of research groups interested in anion binding and sensing. This is because they are relatively easy to synthesize, but mainly because they posses anion-binding properties that are different from that of regular calix[4]pyrroles. The difference in anion-binding properties stems from a different binding mode between the NCCP and anion. While the regular CPs adopt in the complex an ideal cone-like conformation where all four pyrroles-NHs engage in hydrogen bonding to the anion, the inverted pyrroles do not allow forming the cone. NCCPs bind anions via a confused cone (CC), by three NH hydrogen bonds with an anion and a CH–anion contact. This different binding mode results also in different anion-binding affinity and selectivity compared to regular CPs. Also, the inverted pyrroles offer a unique possibility for selective chemical modification of the receptor. The corresponding colorimetric sensors were tested for anion binding applications. The results of colorimetric assays for anions are presented and potential applications discussed.     

Electrostatic ion chromatography of polarizable anions in saline waters with N-[2-[acetyl(3-sulfopropyl)amino]ethyl]-N,N-dimethyldodecanaminium hydroxide (ammonium sulfobetaine-1) as the stationary phase and a dilute electrolytic solution as the mobile phase.

A new type of zwitterionic surfactant, N-{2-[acetyl(3-sulfopropyl)amino]ethyl}-N,N-dimethyldodecanaminium hydroxide (ammonium sulfobetaine-1), with a greater distance between the two charged groups, was used as the stationary phase for electrostatic ion chromatography (EIC) of polarizable anions (e.g., thiocyanate, iodide and nitrate) in saline water samples. The targeted species (polarizable anions) were baseline separated using this type of zwitterionic surfactant as the stationary phase, but the highly polarizable species (iodide and thiocyanate) were eluted faster (compared with the results obtained using N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, C12N3S, with a shorter distance between the two charged groups, as the stationary phase). In other words, the extent of binding of the highly polarizable anion (iodide and thiocyanate) was found to be smaller when using ammonium sulfobetaine-1 as the stationary phase. This provides a rapid but effective method for the analysis of highly polarizable anions in saline water samples. The results for the successful detection of iodide in seawater demonstrates the usefulness of this new type of zwitterionic surfactants for EIC.     

Anion receptors based on ureido-substituted thiacalix[4]arenes and calix[4]arenes

The regioselective nitration of 25,27-dipropoxythiacalix[4]arene was carried out as a key step in the synthesis of thiacalix[4]arene derivative bearing two arylureido functions on the upper rim. The preorganisation of ureido units using the thiacalix[4]arene/calix[4]arene moieties as a molecular scaffold gave novel anion receptors. These compounds, albeit based on hydrogen bonding interactions, show good complexation ability even in highly HB-competitive solvent, such as DMSO. Direct comparison of otherwise identical structures 6a and 7a revealed remarkable dominance of the thiacalix[4]arene derivative over its classical analogue in anion binding.     

Aramidocalix[4]arenes as new anion receptors

Calix[4]arenes bearing aromatic amide (aramid) moieties at the upper rim display interesting recognition properties toward anions and in particular versus planar trigonal nitrate and Y-shaped benzoate. Molecular modeling and DFT calculations indicate that the high affinity displayed by aramidocalix[4]arenes 3 and 4 for nitrate anion is likely due to the planar arrangement of the NH groups, which form an unusual six-hydrogen-bond scheme with nitrate anion.     

Mono Halogen Substituted Calix[4]pyrroles: Fine-tuning the Anion Binding Properties of Calix[4]pyrrole

Abstract

Single halogen atom (i. e. I, Br, Cl and F) substituted calix[4]pyrroles, compounds 2, 3, 4 and 5, were synthesized. Studies of these systems reveal that replacement of a single β-pyrrolic hydrogen atom can increase the anion binding ability of calix[4]pyrroles for a variety of anions (e. g. Cl^?, Br^?, H₂PO₄ ^? and HSO^? ₄) relative to normal non-halogen substituted calix[4]pyrrole 1. In the case of chloride anion, the expected relative affinity sequence of 5 > 4 > 3 > 2 was observed. This was not found to be true for Br^?, H²PO^? ₄, and HSO^? ₄. Here, the chlorine substituted calix[4]pyrrole 4 was found to display a slightly higher affinity in the case of each anion than the fluorine-bearing derivative 5. This was rationalized in terms of intermolecular NH … F hydrogen bonding interactions being present in CD₂Cl₂ solutions of 5. Support for this latter conclusion came from concentration and temperature-dependent NMR spectroscopic studies.

A matched set of mono halogen substituted calix[4]pyrroles was used to study in detail, the extent to which halogen substituents may be used to fine-tune the anion binding properties of calix[4]pyrroles.     

The benzil-cyanide reaction and its application to the development of a selective cyanide anion indicator

The benzil-cyanide reaction is a cyanide-specific reaction that has been exploited to produce a colorimetric indicator for this toxic anion. This was done by producing a pi-extended analogue of benzil, 7, which is soluble in a 70:30 (v/v) mixture of methanol-water. In this medium, dilute solutions of 7 are yellow but produce colorless products when exposed to low concentrations of cyanide anion (> or = 1.7 microM; added as an aqueous NaCN solution), but no other common anions (e.g., OH(-), F(-), N3(-), benzoate(-), and H2PO4(-)). On the basis of these observations and supporting mechanistic analyses, it is concluded that the modified benzil system 7 is a promising cyanide anion indicator that is attractive in terms of its selectivity, ease-of-use, water compatibility, and the low, naked-eye discernible cyanide detection limit it provides.     

A multipurpose compound for protective coatings

International regulations concerning the protection of both the environment and industrial workers have prompted paint manufacturers and end users to look for suitable replacements for traditional pigments.One of the aims of this investigation was to study the inhibitive properties of ferric benzoate-based paints in simulated marine corrosion condition (3% NaCl solution). Anticorrosive paints containing ferric benzoate were formulated and their performance was evaluated by accelerated (salt fog chamber) and electrochemical (corrosion potential and ionic resistance) tests.In addition, the action of ferric benzoate as a probable antifouling agent was studied through laboratory tests and marine exposure. Ferric benzoate hydrolyzes in seawater producing a pH decrease. In this sense, the effects of the pH and the anion benzoate dissolved in seawater and leached from an antifouling paint were evaluated on Balanus amphitrite nauplii. Although benzoate anion showed an intense narcotic effect on nauplii, the results of this research clearly demonstrated that the antifouling activity is due to the combined action of this anion and the pH decrease, i.e. there exists a synergic effect. Field trials confirmed laboratory assays; paints containing ferric benzoate were effective for inhibiting organism settlement and as corrosion of metal.     

Evaluation of the binding ability of tetraaza[2]arene[2]triazine receptors anchoring l-alanine units for aromatic carboxylate anions

The binding affinities of three new tetraaza[2]arene[2]triazine based macrocycles anchoring one (AC1A) and two (AC2A and Me₄AC2A) l-alanine amino acid units for five aromatic carboxylate anions (bz^?, ph^2?, iph^2?, tph^2? and btc^3?) were investigated in DMSO-d₆. ¹H NMR titrations revealed that the AC1A and AC2A receptors exhibit comparable anion affinities, suggesting that the two l-alanine binding units in AC2A are not simultaneously involved in the anion recognition as indicated by molecular dynamics simulations carried out for selected AC1A and AC2A associations using the AMBER force field (GAFF). New force field parameters were developed in order to mimic the structural singularity derived from the N–H macrocyclic bridges.     

Calix[4]pyrrole-capped metalloporphyrins as ditopic receptor models for anions

Strapped calix[4]pyrrole-metalloporphyrin conjugates, potential hosts for anionic guests, have been synthesized and characterized. The condensation unexpectedly resulted in the formation of the two conformational isomers of calix[4]pyrrole-capped porphyrins 6 and 7. The anion binding studies revealed that only isomer 6 showed strong binding with fluoride anion in organic solvent, and neither isomer showed any appreciable binding with Cl-, Br-, and I-. [structure: see text]     

Anion-templated assembly of a [2]catenane

The first example of a [2]catenane structure to be synthesized using anion templation is described. The nature of the anion template is demonstrated to be crucial to the assembly process, with only chloride anion producing the [2]catenane in acceptable yield. Anion binding studies reveal a dramatic catenation effect on anion selectivity properties as compared to a noncatenated acyclic receptor.     

Synthesis of calix[4]arene-cyclen conjugates

Novel calix[4]arene derivatives constrained in the cone or 1,3-alternate conformations, bearing one or two cyclen (1,4,7,10-tetraazacyclododecane) moieties directly connected to the upper rim, have been synthesized using Buchwald-Hartwig coupling reaction. The complexation ability and hydrolytic activities of selected Zn(II) complexes of these calixarenes were studied. Although the attempts to hydrolyze activated phosphodiester bonds were unsuccessful, the NMR titration experiments revealed binding affinity for chloride, acetate, and benzoate anions in defined stoichiometry.     

Halo-derivatised calix[4]tubes

Halo functionalisation of calix[4]tubes has been investigated through both derivatisation of individual calix[4]arenes and calix[4]tubes, using classical synthetic methods, to allow preparation of a series of novel derivatives. The solution and solid state properties are in accordance with the constituent[4]arenes adopting flattened cone arrangements which on complexation with potassium simplify to a regular cone. Electrospray and 1H NMR studies, combined with molecular modelling have been used to ascertain the metal binding of this new series of cryptand like ionophores, demonstrating their retained selectivity for binding potassium over other Group 1 metals and the dependence on counter anion in the weak binding of silver.     

带状修饰的杯[4]吡咯对卤素阴离子识别的研究

应用分子力学／分子动力学模拟方法研究了4种带状修饰的杯[4]吡咯对卤素阴离子的识别作用.模拟结果表明, 加入带状修饰链和调节链长可以增强对某种特殊卤素阴离子的识别, 在这个过程中, 卤素阴离子与杯[4]吡咯NH上的氢形成的多氢键使主体分子与阴离子形成稳定复合物, 同时带状修饰链通过改变杯吡咯空腔大小影响主体分子与卤素阴离子的氢键相互作用, 从而调节了对不同阴离子的识别能力.     

Colorimetric recognition of anions using preorganized tetra-amidourea derived calix[4]arene sensors

The synthesis and the spectroscopic studies of the amidourea based calix[4]arene sensors 1 and 2 are described. The 4-nitrophenyl based sensor 1 was synthesized in two steps from the corresponding calix [4]arene tetraethyl ester and shown to give rise to color changes in the UV-vis spectra in DMSO upon recognition of pyrophosphate and fluoride. Fitting the changes in the absorption spectra using nonlinear regression analysis indicated strong binding of several anions by 1 such as acetate and hydrogen phosphate in 1:1 (Host:Guest) stoichiometry, and at higher concentration in 1:2 stoichiometry. The preorganized calix-cavity was, however, not found to host chlorine while binding of bromide was determined. At high concentrations of these anions, significant colorimetric changes were also observed that were clearly visible to the naked eye for both pyrophosphate and fluoride. The phenyl analogue 2 was made to enable analysis of the anion recognition using 1H NMR titrations and showed that ions such as phosphate were bound in 1:1 stoichiometry, whereas the "urea" protons were shown to be significantly affected upon coordination to the anion.     

Comparison of pillar[5]arene and calix[4]arene anion receptor ability in aqueous media

Abstract

Recognition ability of both cationic pillar[5]arene and calix[4]arene has been studied in aqueous media. Anion complexation can be evaluated from their ability to complex their counterions as well as an added external organic anion. DOSY NMR experiments and fluorescence quenching show that pillararenes have a larger ability for including their own counterions than calixarenes irrespective of the anion (tetrafluoroborate or chloride or bromide) and the structure of the cationic moiety (trimethylammonium or methylimidazolium). Counterion complexation shows a picture where four to five positive charges of the pillar[5]arene are neutralised, meanwhile only one positive charge of the calixarene is neutralised for a 1 mM solution of the macrocycle. Irrespective of the smaller net positive charge in the pillar[5]arene, its binding ability for organic anions (toluenesulfonate or hydroxybenzoate) is larger than for calix[4]arene allowing a better accommodation of the guest in its cavity. The larger separation between the cationic groups of the receptor and its electron-rich aromatic region improves the anion recognition ability for pillar[5]arene.     

Host-guest complexation behaviour of emissive calix[3]naphthobipyrrole toward aromatic guests

A new hexapyrrolic macrocycle composed of three naphthobipyrrole units has been synthesized which exhibited different emission changes with hydrogen pyrophosphate and benzoate ions. Phosphate ions caused red shifting of the fluorescence band of calixnaphthobipyrrole while benzoate ions resulted in quenching of the same. The fluorescence quenching was further utilized for investigating binding preferences of the probe with neutral aromatic guest molecules substituted with groups having varied electron withdrawing abilities. The extent of quenching with the neutral guests increased with the increasing electron deficiency on the aromatic ring of the guest. The fluorescence quenching has been ascribed to electron transfer from the host to the guest species.     

Analytical applications of nano-baskets of calix[4]pyrroles

Calix[4]pyrrole is one such class which holds a great promise in the fields of sensors and their unique behavior as sensors owes to its structural flexibility. Anion binding ability of calix[4]pyrrole has been modified in a variety of ways. Introduction of electron releasing and electron withdrawing groups at the meso position or at β-pyrrolic positions leads to calix[4]pyrrole with deep cavities and fixed walls which shows increased selectivity and modified binding effects. Strapping of calix[4]pyrrole is another way to modify its structural behavior which is responsible for its binding behavior. Choice of strap could play a profound role not only in increasing the intrinsic anion binding affinity of calix[4] pyrrole, but also in modulating the receptor anion stoichiometry, thereby modifying potentially the inherent anion binding selectivity. Calix[n]pyrroles with extended cavities have also been synthesized. Such as calix[3]bipyrrole binds bromide substantially with high affinity than calix[4]pyrrole. Calix[4]pyrrole has also been used to produce anion sensors that can report the presence of anion by means of a color change. The medium effect on the complexation of calix[4]pyrrole and anion has been investigated in various solvents. Calix[4]pyrrole has also been used to increase the ionic conductivity of solid polymer electrolyte by anion complexation of the metal salt. Calix[4]pyrrole has been used to obtain optical sensors using surface plasmon resonance technique. Composite films of cellulose acetate containing calix[4]pyrrole has also been reported which has potential usage in packaging, storage and preservation. In nut shell, calix[4]pyrrole can be modified in a variety of ways to form versatile sensors which can be used in variety of ways in various areas.     

Benzo[A]Pyrene and Benzo[E]Pyrene: Photoreactivity and Phototoxicity toward Human Keratinocytes

Polycyclic aromatic hydrocarbons (PAHs) are a group of organic compounds derived mostly from the incomplete combustion of fossil fuels and biomass. Human skin can absorb PAHs and the uptake increases with their molar mass and lipophilicity. Benzopyrene is high molecular weight PAH frequently appearing in ambient pollution. It exists in two isomeric forms: benzo[a]pyrene (BaP) and benzo[e]pyrene (BeP), which exhibit different biological activity. Although certain properties of benzopyrenes suggested photoreactivity of the compounds, no direct measurements were previously conducted to characterize their photochemical activity. In this study, quantum yield and action spectra of singlet oxygen photogeneration by BaP and BeP were measured by time-resolved near-infrared phosphorescence, and the ability of both compounds to photogenerate superoxide anion was assessed by electron paramagnetic resonance (EPR) spin-trapping. The measurements revealed high efficiency of benzopyrenes to photogenerate singlet oxygen and their ability to photogenerate superoxide anion. Using HaCaT cells as single-layer skin model, we demonstrated concentration-dependent and light-dependent cytotoxicity of BaP and BeP. The compounds induced damage to the cell mitochondria and elevated the levels of intracellular reactive oxygen species.